Devices that locate and measure three dimensional spatial coordinates (x, y, z tuples) by triangulating with projected light beams such as laser light and a camera such as a charge coupled device (CCD) array camera are well known. These devices project a light beam or laser beam in the form of a straight line or a spot moving in a straight line onto the surface to be scanned and form an image of the reflected spot or line of light using the CCD camera. The camera and beam projector are fixed at opposite ends of an interconnecting structure that acts as a base line fixture. The distance between the camera and projector along the base line is fixed and is known and provides a base line for the system. The projector and camera generally have fixed orientation to the base line and in some cases the direction in which the projected light is directed may be rotated about an axis perpendicular to the base line. The angle of the camera's focal axis to the base line fixture is known and the angle of the projected beam to the base line is either known or measured. The projector may project either a line of light extending perpendicular to the base plane (a plane parallel to both the base line and the focal axis of the camera) or a spot of light that is moved at a selected rate along a line perpendicular to the base plane to provide a scan line. This scan line formed on the object reflects light back to the camera to strike the CCD array in the camera at different positions in the array depending on the position of the particular spot the illumination of which is being detected on the array.
It will be apparent that to cover the whole surface area, if the scanning is done say vertically then a plurality of side by side vertical scan lines (horizontally spaced) are successively used to cover the surface of the object being examined. Similarly if the scan lines are horizontal, a plurality of such side by side horizontal lines (vertically spaced) are projected in sequence and measured to obtain data representation of the whole surface of the object being examined.
The spatial resolution attainable with such a device is limited by several factors.
1. The accuracy and resolution of the base line length measurement. PA0 2. The accuracy and resolution of the angular measurement at the initial calibration stage. PA0 3. The accuracy and resolution of successive angular measurements taken as the projector moves the beam across the object being scanned, i.e. the angle of the outgoing beam to the base line. PA0 4. The accuracy and resolution of successive angular measurements taken by the camera of the image of the laser spot (angle of the return beam to the base line) and in particular PA0 5. The ratio of the length of the base line to the shortest distance between the base line and the surface or area being measured, i.e. the stand-off.
Generally, unless the ratio of base line length to stand-off is high (in the order of about one) good resolution in the z direction is very difficult. It will thus be apparent that with the fixture inter-connecting the projecting and receiving stations, the spacing and thus the length of the base line is severely limited and for this reason these systems have normally only been used in relatively close quarters, i.e. base lines up to 1 meter with spacings between the area or surface being examined and the apparatus (the stand off) of up to 2 meters are common.
In all of these devices the exposure time per unit area illuminated or rate of scanning is substantially constant over the complete area being scanned.